Long Message - Pictures with a CMYK colorspace are not supported; source pictures must use a RGB colorspace compatible with all web browsers supported by eBay. Submit pictures created using a camera or scanner set to save images with RGB color.

Resolution:

Some or all of the product images have to be modified accordingly in order to support the above colorspace, that is mentioned in the error. One of the easiest way to fix that for single items is to open each image in the User Interface using the built-in Image Editor in the UI and just click Save, because then the system will save the images in the correct format, that eBay require for JPG images.